Zoology. 1119; 
Every zoologist who has essayed the problem of homology present- . 
ed by the head and the appendages, has made a more or less con- 
spicuous failure, and this, as I am inclined to believe, has resulted 
solely from the fact that there is no true homology in these parts. 
I will not now discuss these points in detail, but will indicate the 
facts and reasons for my views. 
perfect definition should include all the objects. intended to be 
defined, and at the same time exclude all others. Applying this 
we find it all but impossible in few or many words to frame a defi- 
nition which will at once characterize all myriapods, and exclude 
the hexapods, and at the same time take into account structures 
which have any morphological value, The best we can do is some- 
what after this fashion :—Myriapods are air-breathing Arthropods, 
with elongate bodiesand more than three pairs of walking legs. 
Farther than this we cannot go,and even this definition will admit 
Scolopendrella which many now regard as a Thysanure. 
Omitting for the present all mention of these features which 
yriapods have in common, we will take up the points of 
difference between Chilopods and Chilognaths.! The Chilognatha 
(Millepods, galley worms) have a head which bears, besides anten- 
næ, only two pairs of appendages—a pair of jaws or mandibles, and. 
an under-lip composed of the coalesced first maxille. To the head 
succeeds the more or less elongate equally segmented body of which - 
a few anterior segments bear but a single pair of legs, while all the 
rest bear two pairs of appendages, thus apparently affording an ex- 
ception to Savigny’s law that each segment of an Arthropod can . 
bear but a single pair of appendages. The bases of these legs are ° 
placed close to each other, the sternal surface being reduced to an 
extremely narrow plate, or being entirely wanting. 
In the Chilopods, on the other hand, the head bears three pairs 
of mouth-parts, a pair of mandibles and two pairs of maxille while. 
each segment of the body bears but a single pair of walking legs, 
and these are widely separated at their base by the broad sternal 
element. Numerous attempts, as was said above, have been made 
to introduce homology between these two groups in these respects, 
Heathcotes researches show , that that the diplopodous segments 
? Phila Trans. Vol. clxxix., B. (1888). 
of Iulus are in reality double, but they also show that in the head 
there are no traces of more than two pairs of post-oral appendages. 
In the Chilopods the Stigmata which communicate with the 
trachea, are placed at the sides of the body in the thin membrane join- 
ag the dorsal and ventral plates, thus being clearly above and out- 
side the line of the legs; in Scutiger they aredorsal. In the Chilog- 
‘1 Pauropus and the Pauropida are omitted because we know almost 
nothing of their internal structure and their development. | ; 
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